The present invention relates in general to reference antennas for the measurement of emissions from electronic data processing equipment. More particularly, the invention pertains to reference dipole and reference biconical antennas for the measurement of radiated emissions. The invention furthermore relates to a procedure of calibration of other antennas and test sites using the reference antennas. In accordance with the invention the antennas are adapted for the measurement of radiated emissions in the frequency range of 30-1000 MHz.
A reference antenna is one that functions as close to an ideal antenna as possible. An ideal antenna is one whose gain pattern and antenna factor are very close to that predicted in theory. Herein, there are two types of reference antennas that are described, one a dipole antenna and the other a double discone antenna, also known as a biconical antenna.
The traditional biconical antenna covers the frequency range of, say, from 30 to 200 MHz. It is an object of the present invention to provide a new type of biconical antenna which offers broadband, omni directional performance with accurately known VSWR and antenna factors. In accordance with the invention a set of two such biconical antennas may cover a range from 175 to 1000 MHz. They are capable of handling approximately 100 watts for generating strong RF fields for use in susceptibility studies.
It has been found that biconical antennas present a nearly constant impedance at their input terminals between the cones over an extremely wide frequency range. The value of this impedance is determined by the cone angle.
The dipole antenna, at resonance, has a balanced impedance of 73 ohms. To match the 73 ohm impedance to the 50 ohm unbalanced impedance of the coaxial cable, typically found associated with spectrum analyzers and receivers, a balanced to unbalanced transformer, or balun, is used. One of the objects of the present invention is to provide an improved balun that provides for this effective impedance matching with low loss.
In accordance with another feature of the invention there is described herein a technique for the use of the reference antenna to calibrate other antennas. There is also described herein a special tool that is used to allow fast and accurate construction of the antenna and balun. Still in accordance with a further feature of the invention there is described the use of a ferrite sleeve in association with the the balun to reduce unbalanced currents flowing in the shield to thus provide improved performance.